The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to disc drives employing repeatable run-out compensation for write operations.
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position. A multi-rate servo system samples the position of the head relative to a particular track at a particular sampling rate and adjusts the position of the head at a rate that is a multiple of the sampling rate by estimating the position of the head between the measured position samples.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable run-out errors because they cause the same errors each time the head passes along a track. As track densities increase, these repeatable run-out errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in an inner track path that interferes with an outer track path. This is especially acute when a first written-in error causes a head to be outside of an inner track""s ideal circular path and a second written-in error causes the head to be inside of an outer track""s ideal circular path. To avoid limitations on the track pitch, a system is needed to compensate for repeatable run-out errors.
The written-in errors can be compensated by injecting stored compensation values into the servo loop both during read operations and during write operations. However, such a process imposes considerable constraints on the manufacture of the disc drive, such as cost, factory test time and memory size.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.
The present invention relates to disc drives employing repeatable run-out compensation for write operations.
One embodiment of the present invention is directed to a method for improving read error rates in a disc drive employing repeatable run-out compensation for writing but not for reading. Pursuant to the method, a repeatable run-out compensation system which compensates for repeatable run-out during write operations but not during read operations is implemented. After implementing the repeatable run-out compensation system, reader-to-writer offset calibration is performed.
In one embodiment of the present invention, after implementing the repeatable run-out compensation system, read error rates are experimentally determined for a given data track across a range of reader-to-writer offset values. Then a curve representing read error rates as a function of reader-to-writer offset is generated. The optimum reader-to-writer offset is then determined based on the shape of the curve.
Another embodiment of the present invention is directed to a disc drive having a disc, a head, an actuator, a repeatable run-out circuit and a servo controller. The disc is capable of storing data and includes a servo track designed to store servo information that is used to indicate the position of a head relative to the servo track. The head has a writer capable of writing data to the disc and a reader capable of reading data from the disc. The actuator is capable of positioning the head relative to a surface of the disc in response to a servo control signal. The repeatable run-out compensation circuit is capable of compensating for repeatable run-out of the servo track during write operations. The repeatable run-out compensation circuit does not compensate for repeatable run-out of the servo track during read operations. The servo controller is capable of generating a servo control signal based in part on a reader-to-writer offset that is calibrated while the compensation circuit is operational.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.